Extreme-Environment Aircraft Sealants: 3M EC-776, AC-350, AC-770, CS-3204, and PRC/DeSoto PS-700 & PS-872 Compared
There are many types of sealants used on every aircraft, with different compositions and chemical structures for various purposes. They are all built to perform different functions and should not be used interchangeably.
This is especially true for high-temperature sealants that are used in sensitive areas like firewalls, where specialized sealants are not nice to have; they are mandatory. We have compiled a list of some of the most widely used specialty sealants and are giving you the rundown on what they are, what they do, and where to use them, and also a shortcut for military and federal buyers to source these products and buy them now.
If you are ready to buy and you know the product you need for firewall sealant, or any other areas with extreme exposure.
Why Extreme-Environment Sealants Matter in Aircraft Structures
Greenwood Aerospace is your trusted source for parts, components, and, in particular, expendable materials such as sealants and adhesives. Sealants are not cosmetic materials but mission-critical consumables that protect airframes, fuel systems, and internal structures from the roughest environmental conditions worldwide. Failures in sealants here can be catastrophic; think about a sealant failing in a firewall. That can be the difference between moments to work on an emergency and seconds.
In extreme areas of the aircraft, namely engine bays, the performance margins for sealants are narrow. A material can look exactly the same to the naked eye and perform adequately in a controlled hangar environment for curing, but then fail when it is subjected to sustained thermal cycling, exposure to fuels, oil, and hydraulic fluids, allowing moisture contamination
Defining “Extreme Environment” for Sealants
For aircraft and other aerospace vehicles, “extreme environment” refers to any operating condition that places sustained thermal, chemical, mechanical, or environmental stresses.
Temperature extremes from roughly -65°F up to 250°F-400°F, with short-term exposure as high as 2000°F in firewall and fire-zone areas, are what these chemicals must be able to work around or
The products we are about to detail are tailored for the aviation industry and are designed for sealing and gasket jobs, where common causes of failure are, again, fuel exposure, high heat, and chemicals that can work their way into passages in sensitive areas if the sealants do not hold up to the conditions.
Let’s move along and take a look at the different products and what they do, and how they do it.
The Product Lineup and Roles
How These Products Complement, Not Compete
The products listed above are designed for specific aircraft zones and are intended to complement one another, not compete.
Fuel tanks & wetted areas: AC-350 or CS3204 provide structural fuel sealing, while EC-776 adds a protective fuel-resistant coating layer.
Fuselage & pressurized structures: AC-770 handles lightweight fuselage sealing where continuous fuel immersion is not present.
Firewalls & fire zones: PS 700 is specified solely for high-temperature fire resistance and vapor sealing.
Lightning-strike paths & corrosion-prone joints: PS 872 ensures electrical continuity and corrosion protection without compromising sealing.
Sealant Chemistries and Specification Anchors
The sealants in this section are all actually built from three different “buckets” of chemical bases.
- Fuel-resistant elastomer coatings (EC-776): thin, brush/flow-applied barrier layers used to protect substrates and underlying sealants from fuel and chemical exposure.
- Polysulfide sealants (AC-350, AC-770, CS3204, PS 872): two-part aircraft sealants engineered for fuel resistance, weathering, and long-term flexibility in fuselage and tank environments.
- High-temperature firewall sealants (PS 700): specialized synthetic rubber compounds designed to block air/vapor passage and withstand extreme fire-zone exposure.
For safe handling and shop-floor controls, always consult each product’s Safety Data Sheet (SDS/MSDS) before mixing or applying (ventilation, PPE, exposure limits, solvent handling, etc.). Each TDS explicitly directs users to review the SDS/MSDS before use.
If you are ready to buy now, take a look at the table below:
Let’s take a look at the chemistry of each of these products.
EC-776 Fuel-Resistant Elastomer Coating (3M)
Solvent-based, oil-resistant elastomer coating with ~24% solids by weight. EC-776 is a general-purpose, solvent-based adhesive/coating based on an oil-resistant elastomer, with ~24% solids (by weight).
Transparent amber film, applied by brush or flow/spread application (not an extrusion gun product). EC-776 is a thin, syrup-like coating (not a paste sealant), and the TDS specifies application by brush or flow applicator, including fill-and-drain tank coating methods.
Meets MIL-D-17951E sealing compound requirements. EC-776 meets the MIL-D-17951E sealing compound requirements, making it a fully compliant sealant for military maintenance.
Designed to resist oil, gasoline, and aromatic fuels (with significant use limitations)
The “Features” section explicitly calls out resistance to oil, gasoline, and aromatic fuels. The same TDS also notes it is not recommended for alcohol-containing fuels (gasohol) or jet fuel containing microbial organisms—worth flagging as a selection constraint when matching “fuel, jet fuel, chemicals” requirements.
AC-350 Intermediate-Density Polysulfide (3M)
3M AC-350 Class B is a two-component, manganese-dioxide-cured polysulfide sealant that is engineered for fuel tank and fuselage sealing. As a solvent-free, aircraft-grade sealant, it cures at room temperature to form a resilient, elastomeric material capable of accommodating high vibration, thermal cycling, and structural movement.
AC-350 is defined as an intermediate-density polysulfide and cures to a dark gray appearance. This density allows it to maintain outstanding tooling characteristics while still offering the durability and fuel resistance for integral fuel tanks. AC-350 is explicitly approved for sealing and repairing fuel tanks, fuselage seams, and myriad other components that are exposed to fuel and other caustic fluids.
Chemically, AC-350 has excellent resistance to aviation gasoline, jet fuel, and a wide range of petroleum-based products and aircraft chemicals. It’s a common choice for wetted areas and areas where long-term fuel exposure is expected. When mixed, the compound forms a thixotropic paste that can be applied with an extrusion or injection gun for production work or with a spatula for localized repairs and fillet formation, depending on joint geometry.
AC-770 Low-Density Fuselage Polysulfide (3M)
3M AC-770 Class B is a fast-curing, low-density polysulfide sealant designed for fuselage sealing and void filling, not continuous fuel immersion. Like AC-350, it’s a two-component, manganese dioxide-cured polysulfide. Still, the formulation is lower in density and cures faster, supporting weight-sensitive and schedule-driven aircraft maintenance and production environments.
The base compound of AC-770 is off-white, combined with a brown accelerator, and cures to a dark brown elastomeric seal. This visual contrast helps the maintenance team confirm proper mixing and curing during application. AC-770 is intended for sealing fuselage skins, filling voids, and sealing gaps where fuel exposure may be intermittent rather than continuous.
Despite its lower density, AC-770 still exhibits strong resistance to aviation gasoline, jet fuel, water, and standard aircraft chemicals. It’s supplied as a thixotropic paste and is applied with extrusion or injection guns or by spatula, like other Class B polysulfides. Having multiple Class B work-life options allows the technician to tailor cure time to the task at hand, making AC-770 ideal for fast-paced maintenance and assembly operations.
CS-3204 Class B, Type II Fuel-Tank Sealant (3M)
CS-3204 is a two-component polysulfide fuel tank sealant from Flamemaster under the Chem-Seal product line. This sealant cures at room temperature to a flexible rubber specifically designed for long-term exposure to aircraft fuels, lubricants, oils, water, and environmental weathering.
Classified as a Class B, Type 2 sealant under AMS-S-8802, CS-3204 is used for sealing integral fuel tanks and pressurized aircraft cabins. The chemistry is designed to resist sulfur compounds in jet fuels, making it ideal for long-term fuel-immersion environments. When applied correctly, CS-3204 has strong adhesion to aluminum, magnesium, titanium, steel, and other common aircraft substrates, provided surface preparation requirements are strictly followed.
CS-3204 Class B is supplied as a thixotropic paste that will not sag or flow on vertical or overhead surfaces. This makes it ideal for controlled application with extrusion guns in production and for hand-tooling with a spatula in maintenance and repair operations. The predictable flow and tooling characteristics are why it’s often specified for both new-build and in-service fuel-system sealing.
P/S 700 Primerless Firewall Sealant (PRC-DeSoto / PPG)
P/S 700 is a two-part synthetic rubber firewall sealant developed by PRC-DeSoto (PPG Aerospace) for high-temperature fire zone applications. The uncured material is a low-sag paste that can be applied with an extrusion gun or spatula and can be used on vertical and complex firewall geometries without slumping.
One of the key features of P/S 700 is its primerless formulation. In many firewall sealing applications, this simplifies maintenance by reducing the number of steps. While it still requires proper cleaning and surface conditioning. Once cured, P/S 700 forms a resilient elastomeric barrier that prevents air and vapors from passing through firewall structures.
From a performance standpoint, P/S 700 is rated for continuous service from -65°F to 400°F and is qualified to withstand flash-flame exposure up to 2000°F. Testing is conducted in accordance with MIL-S-38249 Type I methods, including flame-resistance testing that demonstrates no flame penetration under prolonged high-temperature exposure. This differs from fuel tank sealants, positioning P/S 700 as a purpose-built firewall and fire-zone material.
P/S 872 Conductive Lightning-Strike Sealant (PRC-DeSoto / PPG)
P/S 872 Class B is a two-part, aluminum-filled, manganese dioxide-cured polysulfide sealant that seals and conducts. Supplied as a low-sag, thixotropic paste, P/S 872 is for applications where corrosion protection and electrical continuity are as important as environmental sealing.
The aluminum filler in P/S 872 allows the cured sealant to conduct electricity across mating surfaces, thereby maintaining lightning strike protection and bonding continuity. In addition to being conductive, the sealant is a corrosion-inhibiting barrier that protects aluminum alloys and interfaces between dissimilar metals from moisture-driven degradation.
According to the datasheet, P/S 872 maintains electrical continuity even under high torque, a requirement that sets it apart from non-conductive polysulfide sealants. Therefore, P/S 872 is specified for lightning strike paths, fastener interfaces, bonded joints, and other structural areas where metal-to-metal continuity is critical to aircraft safety and certification requirements.
Temperature Range and Thermal Behavior
Note: All of this data is taken directly from the manufacturer’s technical data sheets.
Extreme-environment aircraft sealants are defined by how they behave under temperature stress as much as by their chemical resistance. Across fuel tanks, fuselages, firewalls, and lightning-strike paths, sealants must withstand prolonged low temperature exposure at altitude, high temperatures near engines and bleed-air systems, and short-duration thermal excursions during abnormal or emergency conditions. The following products represent different engineering approaches to thermal durability, each tested and qualified to specification.
Continuous and Short-Term Service Temperatures
3M EC-776 Fuel Resistant Coating is designed for low-temperature fuel environments where flexibility and adhesion retention are critical. According to the data sheet, EC-776 coated aluminum panels were immersed for 48 hours in MIL-H-3136 Type III hydrocarbon test fluid at 100°F, then exposed to −65°F for 2 hours and mechanically bent around a mandrel at that temperature. No cracking, checking, or loss of adhesion was observed, demonstrating low temperature flexibility even after fuel exposure.
While EC-776 is air-dried, the data sheet also notes that heating for 1 hour at 250°F can increase moisture resistance. This optional thermal exposure does not make EC-776 a high-temperature sealant but does improve performance in humid or water-prone environments, such as fuel tanks and internal wetted cavities.
For polysulfide sealants, thermal capability is more directly related to structural performance. 3M AC-350 Class B is rated for continuous service from −65°F to +250°F with short-term exposure up to approximately +360°F. These limits are supported by thermal rupture resistance testing, in which AC-350 passed 30 minutes at 300°F and is suitable for high-heat fuel tank and fuselage applications where transient temperature spikes may occur during operation or maintenance.
Low-density fuselage sealants like 3M AC-770 Class B have the same continuous service range of −65°F to +250°F. Still, they are not designed for the same short-duration high-temperature exposure as intermediate-density tank sealants. Instead, AC-770 is focused on thermal flexibility and dimensional stability under repeated temperature cycling. Low temperature flexibility testing showed no cracking, checking, or adhesion loss at −65°F.
High-temperature firewall sealants are in a different thermal category altogether. PRC-DeSoto P/S 700 is rated for continuous service from −65°F to +400°F and is qualified to withstand flash temperatures of 2000°F. Flame testing under MIL-S-38249 Type I methods showed no flame penetration after 15 minutes at 2,000°F, and thermal rupture resistance was maintained before and after flame exposure. This performance profile differs from that of fuel tank or fuselage sealants and anchors the use of firewall sealants in fire-zone structures.
Conductive polysulfide sealants, such as PRC-DeSoto P/S 872 Class B, fall between. P/S 872 is rated for continuous service from −67°F to +250°F with intermittent excursions up to 275°F. The material maintains electrical continuity under these thermal conditions and exhibits low-temperature flexibility at −65°F without cracking or loss of adhesion, making it suitable for lightning strike paths and corrosion-prone structural joints.
Procurement of High Temperature and Firewall Sealants from Greenwood Aerospace
One of the biggest headaches in military aircraft maintenance is securing the materials you need at competitive prices from a reliable, reputable distributor. Once you find a distributor that you can trust, you will never go back.
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